Luffy doing a solo against Kaido and winning is also headcanon.
Except that's where the author is going with it literally this chapter lol.
Just like Zoro cutting Kaido wasn't headcanon but ZKK actually is headcanon. We can believe what we are seeing or explained until it's disproven.
Right now, Oda is removing Zoro from the roof. He didn't have to do this to maintain the idea that Zoro fights the Yonko with Luffy. Luffy is now saying he will beat Kaido, just like he said he will beat Crocodile, Enel, Lucci, Moria, Doflamingo and Katakuri.
So yeah, maybe he is wrong, but I have far more reasons to believe he will be right, EVEN if he loses this second round, because it's Oda's nature to give Luffy his own merited struggle and 1v1.